The present invention relates to a marine propeller unit of the kind arranged to be mounted on the underside of a water-going vessel and to be connected by means of a vertically extending shaft to a driving machinery arranged within said vessel. Propeller units of this kind are becoming more and more common in special-duty vessels requiring a high degree of manoeuverability and thrust control, e.g. such vessels as tugs, ferries, floating cranes, drill rigs, drill ships, pipe-laying ships, cable-laying ships etc.
Such a propeller unit is normally called a thruster, and one known embodiment comprises a streamlined gear housing in which a substantially horizontal propeller shaft is journalled, the propeller being mounted on said shaft. The gear housing includes a conical gearing with the gear wheel or gear ring mounted on the propeller shaft and the drive mounted on the lower end of a substantially vertical drive shaft. The gear housing is carried by a hollow strut which is connected at its lower end to the gear housing, while the upper end of the strut is mounted to the underside of the vessel. The strut is often arranged for rotation relative to the hull of the vessel, about an axis which coincides with the substantially vertical drive shaft of the propeller unit. The upper end of the drive shaft is arranged for connection to a drive engine for driving the propeller. When the propeller unit is one in which the strut is arranged to be rotatable, the upper end of the strut is arranged for connection to a steering mechanism for rotating the propeller unit.
When it is desired to construct a propeller unit of this kind in a manner such as to obtain a high-power output, a particular problem is encountered, insomuch as in order to meet this requirement the conical gearing between the vertical drive shaft and the horizontal propeller shaft must be of correspondingly larger diameter. An increase in the diameter of the conical gear wheel mounted on the propeller shaft requires a corresponding increase in the diameter of the gear housing. The gear housing, which normally has the form of a rotation-symmetrical streamline housing arranged coaxially with the propeller, influences the propeller flow and therewith the efficiency of the propeller. In order to ensure in the best possible manner that the inherent propeller efficiency is maintained, the disturbance of the propeller flow must be as small as possible, which means that the diameter of the gear housing should not be excessively large. The problem of maintaining the diameter of the gear housing within acceptable limits becomes progressively more difficult the larger the diameter of the conical gear wheel becomes, in the attempt to increase the power output of the propeller unit.